1. Field of the Invention
The present invention relates to a brake negative pressure control apparatus and method, and an engine control unit for an internal combustion engine of an in-cylinder direct fuel injection type which is configured to set a combustion mode to a stratified combustion or a homogeneous stoichiometric combustion in accordance with an operating condition of the internal combustion engine, and is equipped with a brake booster for increasing a braking force of a brake with a negative pressure introduced from an intake pipe.
2. Description of the Prior Art
A conventional control apparatus of the type mentioned above is disclosed in Japanese Patent No. 3003528. An internal combustion engine disclosed in this patent is an internal combustion engine of an in-cylinder direct fuel injection type which directly injects a fuel into cylinders. Specifically, the internal combustion engine injects a fuel in a compression stroke with a throttle valve being substantially fully opened, when the engine is lightly loaded, to perform a stratified combustion at an air/fuel ratio extremely leaner than the stoichiometric air/fuel ratio to improve the fuel economy and the like. In other operation regions, the engine injects the fuel in an intake stroke to perform a homogeneous combustion with an air/fuel mixture at the stoichiometric air/fuel ratio to ensure the power. The control apparatus utilizes a negative pressure within an intake pipe to detect a negative pressure within a brake booster for increasing a braking force of a brake, and forcedly controls the opening of the throttle value (hereinafter called the xe2x80x9cthrottle valve openingxe2x80x9d) in a closing direction from a reference opening by a correction opening when the detected negative pressure is lower than a predetermined negative pressure. The reference opening is set in accordance with an operating condition of the internal combustion engine, while the correction opening is set in accordance with the rotational speed and injected fuel amount of the internal combustion engine. When the throttle valve is controlled in the closing direction by the correction opening in the foregoing manner, the control apparatus sets the combustion mode to a homogeneous combustion mode for ensuring the ignition, and calculates fuel injection parameters (a fuel injection time, an ignition timing, and an opening of an EGR valve) for the homogeneous combustion mode.
The conventional control apparatus described above, however, implies the following problems. When a low negative pressure is detected within the brake booster, the control apparatus forcedly sets only the throttle valve opening in the closing direction, among other fuel injection parameters which have been previously set for the homogeneous combustion mode, throwing the fuel injection parameters out of balance to adversely affect the fuel economy, drivability, and exhaust gas characteristic. To eliminate such an inconvenience, if the fuel injection parameters for the homogeneous combustion mode are set to different values when the throttle valve opening is forcedly moved in the closing direction from those when the throttle opening is not so controlled, a complicated control is required to cause evil influences such as a requirement for a larger capacity of memory, an increased calculation time, and the like.
The present invention has been made to solve the problems as mentioned above, and it is an object of the invention to provide a brake negative pressure control apparatus and method, and an engine control unit for an internal combustion engine which is capable of ensuring a negative pressure within a brake booster and a stable combustion while avoiding a complicated control.
To achieve the above object, according to a first aspect of the invention, there is provided a brake negative pressure control apparatus for an internal combustion engine of an in-cylinder direct fuel injection type which is configured to set a combustion mode to a stratified combustion or a homogeneous stoichiometric combustion based on a required output in accordance with at least an opening of an accelerator pedal and an operating condition of the internal combustion engine, and is equipped with a brake booster for increasing a braking force of a brake with a negative pressure introduced from an intake pipe.
The brake negative pressure control apparatus according to the first aspect of the invention is characterized by comprising negative pressure detecting means for detecting a negative pressure within the brake booster; combustion mode setting disabling means for disabling the combustion mode to be set to the stratified combustion when the detected negative pressure is lower than a first predetermined negative pressure; and control means for controlling a throttle valve opening in accordance with a target throttle valve opening in a homogeneous combustion mode when the combustion mode setting disabling means disables the combustion mode to be set to the stratified combustion.
According to this brake negative pressure control apparatus for an internal combustion engine, when the negative pressure within the brake booster is lower than the first predetermined negative pressure, the combustion mode is disabled to be set to the stratified combustion, i.e., the combustion mode is set to the homogeneous stoichiometric combustion. Thus, the negative pressure within the brake booster introduced from the intake pipe can be ensured by controlling the throttle valve opening based on the target throttle valve opening in accordance with the homogeneous combustion mode to maintain the negative pressure within the intake pipe. In addition, a stable combustion can be provided because of the combustion mode set to the homogeneous stoichiometric combustion. Unlike the conventional control apparatus, when an insufficient negative pressure is prevailing in the brake booster, a normal control in the homogeneous stoichiometric combustion is applied as it is to automatically close the throttle valve in sequence based on this control. Since this eliminates the need for forcedly closing the throttle valve, a complicated control can be avoided.
To achieve the above object, according to a second aspect of the invention, there is provided a brake negative pressure control method for an internal combustion engine of an in-cylinder direct fuel injection type which is configured to set a combustion mode to a stratified combustion or a homogeneous stoichiometric combustion based on a required output in accordance with at least an opening of an accelerator pedal and an operating condition of the internal combustion engine, and is equipped with a brake booster for increasing a braking force of a brake with a negative pressure introduced from an intake pipe.
The brake negative pressure control method according to the second aspect of the invention is characterized by comprising the steps of detecting a negative pressure within the brake booster; disabling the combustion mode to be set to the stratified combustion when the detected negative pressure is lower than a first predetermined negative pressure; and controlling a throttle valve opening in accordance with a target throttle valve opening in a homogeneous combustion mode when the combustion mode is disabled to be set to the stratified combustion.
This brake negative pressure control method provides the same advantageous effects as described above concerning the brake negative pressure control apparatus according to the first aspect of the invention.
To achieve the above object, according to a third aspect of the invention, there is provided an engine control unit including a control program for causing a computer to carry out control of a brake negative pressure for an internal combustion engine of an in-cylinder direct fuel injection type which is configured to set a combustion mode to a stratified combustion or a homogeneous stoichiometric combustion based on a required output in accordance with at least an opening of an accelerator pedal and an operating condition of the internal combustion engine, and is equipped with a brake booster for increasing a braking force of a brake with a negative pressure introduced from an intake pipe.
The engine control unit according to the third aspect of the invention is characterized in that the control program causes the computer to detect a negative pressure within the brake booster; disable the combustion mode to be set to the stratified combustion when the detected negative pressure is lower than a first predetermined negative pressure; and control a throttle valve opening in accordance with a target throttle valve opening in a homogeneous combustion mode when the combustion mode is disabled to be set to the stratified combustion.
This engine control unit provides the same advantageous effects as described above concerning the brake negative pressure control apparatus according to the first aspect of the invention.
Preferably, in the brake negative pressure control apparatus for an internal combustion engine, the combustion mode further includes a homogeneous lean combustion, wherein the combustion mode setting disabling means disables the combustion mode to be set to the stratified combustion and to the homogeneous lean combustion when the negative pressure is lower than the first predetermined negative pressure.
According to this preferred embodiment of the brake negative pressure control apparatus, when the negative pressure within the brake booster is lower than the first predetermined negative pressure, the combustion mode setting disabling means disables the combustion mode to be set to the stratified combustion and to the homogeneous lean combustion, so that the combustion mode is set to the homogeneous stoichiometric combustion. Since a negative pressure within the intake pipe is also low in the homogeneous lean combustion, the homogeneous lean combustion is disabled, and the combustion mode is set to the homogeneous stoichiometric combustion. Thus, a negative pressure can be maintained in the intake pipe to ensure a negative pressure within the brake booster and achieve a stable combustion without fail.
Preferably, in the brake negative pressure control method for an internal combustion engine, the combustion mode further includes a homogeneous lean combustion, wherein the step of disabling setting of the combustion mode includes disabling the combustion mode to be set to the stratified combustion and to the homogeneous lean combustion when the negative pressure is lower than the first predetermined negative pressure.
This preferred embodiment of the brake negative pressure control method provides the same advantageous effects as provided by the corresponding preferred embodiment of the brake negative pressure control apparatus.
Preferably, in the engine control unit, the combustion mode further includes a homogeneous lean combustion, wherein the control program causes the computer to disable the combustion mode to be set to the stratified combustion and to the homogeneous lean combustion when the negative pressure is lower than the first predetermined negative pressure.
This preferred embodiment of the engine control unit provides the same advantageous effects as provided by the corresponding preferred embodiment of the brake negative pressure control apparatus.
Preferably, in the brake negative pressure control apparatus for an internal combustion engine, the combustion mode setting disabling means releases the disabled combustion mode when the negative pressure is increased to be higher than the first predetermined pressure which is higher than the first predetermined negative pressure while the combustion mode setting disabling means disables the combustion mode to be set to the stratified combustion and to the homogeneous lean combustion.
According to this preferred embodiment of the brake negative pressure control apparatus, the combustion mode setting disabling means releases the disabled combustion mode when the negative pressure is increased to be higher than the second predetermined pressure, while the combustion mode setting disabling means disables the combustion mode to be set to the stratified combustion and to the homogeneous lean combustion, on the assumption that the negative pressure within the brake booster has been recovered to such an extent that it can amplify the braking force of the brake. It is therefore possible to minimize an exacerbated fuel economy caused by disabling the stratified combustion and the like. In addition, the combustion mode can be stably switched without causing hunting by setting the second predetermined negative pressure larger than the first predetermined negative pressure.
Preferably, in the brake negative pressure control method for an internal combustion engine, the step of disabling setting of the combustion mode includes releasing the disabled combustion mode when the negative pressure is increased to be higher than the first predetermined pressure which is higher than the first predetermined negative pressure while the combustion mode is disabled to be set to the stratified combustion and to the homogeneous lean combustion.
This preferred embodiment of the brake negative pressure control method provides the same advantageous effects as provided by the corresponding preferred embodiment of the brake negative pressure control apparatus.
Preferably, in the engine control unit, the control program further causes the computer to release the disabled combustion mode when the negative pressure is increased to be higher than the first predetermined pressure which is higher than the first predetermined negative pressure while the combustion mode is disabled to be set to the stratified combustion and to the homogeneous lean combustion.
This preferred embodiment of the engine control unit provides the same advantageous effects as provided by the corresponding preferred embodiment of the brake negative pressure control apparatus.
Preferably, in the brake negative pressure control apparatus for an internal combustion engine, the combustion mode setting disabling means continues to disable the stratified combustion and the homogeneous lean combustion when the required output is larger than a predetermined upper limit output after the negative pressure is increased beyond the second predetermined negative pressure.
According to this preferred embodiment of the brake negative pressure control apparatus, even if the negative pressure is recovered, the combustion mode setting disabling means continues to disable the combustion modes, given a higher priority to the output torque of the engine, thereby making it possible to set a combustion mode suitable for an operating condition of the engine.
Preferably, in the brake negative pressure control method for an internal combustion engine, the step of disabling setting of the combustion mode includes continuously disabling the stratified combustion and the homogeneous lean combustion when the required output is larger than a predetermined upper limit output after the negative pressure is increased beyond the second predetermined negative pressure.
This preferred embodiment of the brake negative pressure control method provides the same advantageous effects as provided by the corresponding preferred embodiment of the brake negative pressure control apparatus.
Preferably, in the engine control unit, the control program further causes the computer to continuously disable the stratified combustion and the homogeneous lean combustion when the required output is larger than a predetermined upper limit output after the negative pressure is increased beyond the second predetermined negative pressure.
This preferred embodiment of the engine control unit provides the same advantageous effects as provided by the corresponding preferred embodiment of the brake negative pressure control apparatus.